Adding code to

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Problem :

I’m taking a look at how the model system in django works and I noticed something that I don’t understand.

I know that you create an empty file to specify that the current directory is a package. And that you can set some variable in so that import * works properly.

But django adds a bunch of from … import … statements and defines a bunch of classes in Why? Doesn’t this just make things look messy? Is there a reason that requires this code in

Solution :

All imports in are made available when you import the package (directory) that contains it.



import something


import dir
# can now use dir.something

EDIT: forgot to mention, the code in runs the first time you import any module from that directory. So it’s normally a good place to put any package-level initialisation code.

EDIT2: dgrant pointed out to a possible confusion in my example. In import something can import any module, not necessary from the package. For example, we can replace it with import datetime, then in our top level both of these snippets will work:

import dir


import dir.some_module_in_dir

The bottom line is: all names assigned in, be it imported modules, functions or classes, are automatically available in the package namespace whenever you import the package or a module in the package.

It’s just personal preference really, and has to do with the layout of your python modules.

Let’s say you have a module called erikutils. There are two ways that it can be a module, either you have a file called on your sys.path or you have a directory called erikutils on your sys.path with an empty file inside it. Then let’s say you have a bunch of modules called fileutils, procutils, parseutils and you want those to be sub-modules under erikutils. So you make some .py files called,, and


Maybe you have a few functions that just don’t belong in the fileutils, procutils, or parseutils modules. And let’s say you don’t feel like creating a new module called miscutils. AND, you’d like to be able to call the function like so:

rather than doing

So because the erikutils module is a directory, not a file, we have to define it’s functions inside the file.

In django, the best example I can think of is django.db.models.fields. ALL the django *Field classes are defined in the file in the django/db/models/fields directory. I guess they did this because they didn’t want to cram everything into a hypothetical django/db/models/ model, so they split it out into a few submodules (,, for example) and they stuck the made *Field definitions in the fields module itself (hence,

Using the file allows you to make the internal package structure invisible from the outside. If the internal structure changes (e.g. because you split one fat module into two) you only have to adjust the file, but not the code that depends on the package. You can also make parts of your package invisible, e.g. if they are not ready for general usage.

Note that you can use the del command, so a typical may look like this:

from somemodule import some_function1, some_function2, SomeObject

del somemodule

Now if you decide to split somemodule the new might be:

from somemodule1 import some_function1, some_function2
from somemodule2 import SomeObject

del somemodule1
del somemodule2

From the outside the package still looks exactly as before.

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